A conventional apparatus for forming a continuous elastomeric strip, such as in the nature of a tire tread slab may utilize an assembly constituted primarily of a driven roller and a stationary die-blade confronting the roller and defining therewith a convergent nozzlelike chamber. The chamber operates on an elastomeric mass to convert the latter into a continuous elastomeric strip having thin, longitudinally extending edges.
Conventional practices for shaping or otherwise fabricating elastomeric strips, in the form of tread slabs of rubber composition, for use in manufacturing pneumatic tires, have proven to be less than most desirable, this because the tread slab, at least in certain instances, is not reliably formed with coherent edges in which the integrity thereof is sharply maintained. Edges, as conventionally understood, are those opposed, longitudinal, elongate edges of the tread slab which are of extremely thin nature, and present a smooth transition between the sidewall and tread portions of a pneumatic tire carcass to permit high quality stitching (adhesion) of the tread slab to the tire carcass.
A tread slab, at least pursuant to one conventional apparatus, may be extruded under pressure through a slit-die, the latter being contoured to appropriately impart to the elastomeric material, of which the tread slab is constituted, a preferred cross-sectional contour. The disadvantage associated with the conventional slit-die assembly is that the pressure exerted against the elastomeric material, as the latter issues through the slit-die, is a limited function of the pressure developed by the extrusion assembly upstream of the slit-die. As a result, therefore, since the conventional extrusion apparatus, upstream of the slit-die, has a limited maximum capacity for developing pressure therein especially so as not to overplasticize the elastomeric mass, the apparatus cannot reliably impart to the elastomeric material the more preferable, coherent and generally blemish-free edges.
In order to overcome the disadvantages associated with the conventional slit-die assembly, the slit-die, downstream of the extrusion unit, has been replaced by, or otherwise substituted for, a calendering assembly in which there are provided a pair of opposing rolls or rollers, one of which is contoured and cooperates with an uncontoured, substantially cylindrical surface of the opposing roller, or mate thereof, for purposes of imparting to the elastomeric mass, as the latter is extruded to and through the nip area or clearance therebetween, a preferred cross-sectional appearance.
The calender rolls, downstream of the extruder in an additive or supplemental manner, provide means for increasing the pressure to which the elastomeric mass is subjected as it is extruded through the nip area between the rollers. The pressure is increased from that pressure developed by the extrusion apparatus, to an added or supplemental pressure generated by the rollers as the latter are drivingly rotated in the direction of feed or extrusion of the elastomeric mass. Thus, the utilization of calender rolls downstream of a conventional extrusion apparatus is somewhat more beneficial than merely utilizing a conventional slit-die assembly because the calender rolls themselves, when driven, introduce increased energy into the elastomeric mass as the latter passes through the nip area therebetween.
However, calendering rolls provide only a minimum effective surface area against which the elastomeric mass engages, the effective or working surface area being respective segments of the opposing rolls of minimal arcuate extent, which converge toward and thereafter diverge away from one another specifically at the nip area between the rolls. Thus, although calender rolls permit the increasing of nip pressures such as by decreasing the clearance between the rolls, or by increasing the rate of rotation of the rolls, since the elastomeric mass is only subjected to the increased pressure over minimal arcuate working surface segments of the opposing rolls, the ultimate cross-sectional contour of the elastomeric product issuing from the nip will not necessarily and predictably have blemish-free, highly coherent edges to the extent that may be desirable without replacing/recutting the roll.
Another disadvantage associated with the utilization of calendering rolls, one which is of substantially cylindrical extent, or uncontoured, and the other contoured circumferentially to cooperate with the uncontoured roll, is that the contoured roll cannot be most effectively fabricated with a sharp profile, or effectively scraped and cleaned when necessary to present a sharp profile. Thus, the ultimate shape of the elastomeric product is often undesirably dependent upon the lack of a sharp profile in the contoured roll due to fabrication deficiencies or excess elastomeric stock caked upon, and not fully removed from, the contoured roll. Moreover, the contoured roll is of considerable bulk and does not readily lend itself to be easily exchanged for another when necessary for altering the cross-sectional appearance of the elastomeric product.
Thus, despite the provision of calendering rolls downstream of an extrusion assembly for enhancing the degree of pressure to which the elastomeric mass is to be subjected, this being a significant improvement over the conventional slit-die assembly, there still remains serious disadvantages associated with the use of conventional calendering rolls in conjunction with a conventional extrusion apparatus.
Consequently, it had been desirable to provide an improved extrusion die-calendering apparatus in which not only can the pressure, to which the elastomeric mass is subjected, be significantly increased, but the duration, or preservation, of time in which the elastomeric mass is subjected to the increased pressure will be likewise significantly increased to permit the formation or shaping of an elastomeric product, in the nature of a tread slab for pneumatic tires, which will present coherent edges of high integrity. The conventional improved assembly comprises only one roll while the second contoured roll being replaced by a less bulky, readily changeable, stationary, contoured die-blade which can be more accurately fabricated with a sharp profile than can the contoured roll.
It had also been desirable to reduce the extent of swelling normally attributable to an elastomeric product after the product has been discharged from an extrusion-shaping assembly. In this respect, the development of internal stresses within the elastomeric product, during formation thereof, comes about partly from extruder speed and/or roll speed. Further, the extrusion speed of the extrusion die-calendering apparatus may be varied (increased) without affecting the integrity or coherency of the ultimate edges formed on the product, but may affect the profile and shrinkage characteristics of the elastomeric product. Additionally, secondary dies may be utilized upstream of the main die such as for purposes of adjusting or controlling the quantity of elastomeric mass fed to the main die.
The conventional assembly may comprise a driven roller, a stationary die-blade, which confronts and converges toward an arcuate segment of the roller, the die-blade and roller cooperatively defining a pressure chamber therebetween terminating in a narrow restriction orifice, and means such as an extruder for introducing the elastomeric mass under pressure into the chamber and upon the roller. The chamber may gradually reduce the cross-sectional thickness of the elastomeric mass and preform the elastomeric mass, whereas the restriction orifice, under increased pressure, imparts to the elastomeric mass a final cross-sectional construction.